1. Field of the Invention
This invention relates generally to frequency and/or timing acquisition in a wireless communications system, and, more specifically, reliable burst detection as an antecedent to frequency and/or timing acquisition in a wireless communications system.
2. Related Art
In wireless communications systems, a recipient of a wireless transmission, be it a mobile station or a land station or some other element, must become aware of the timing of and frequency with which the transmission occurred in order to decipher the information in the transmission. Such a process is often referred to as the recipient “acquiring” the frequency or timing of the transmission.
Conventional approaches for timing and/or frequency acquisition include closed loop techniques such as Symbol Timing Recovery (STR) or Automatic Frequency Control (AFC). In these techniques, a transmission is prefaced with a preamble, and the recipient acquires frequency and/or timing by analyzing the preamble.
The problem with these techniques is that the acquisition process can take an inordinate amount of time if there is a large frequency offset between the initial assumed frequency and the actual frequency of the transmission, or there is a large timing error between the initial assumed timing and the actual timing of the transmission. Another problem is that, because data cannot be interpreted until acquisition occurs, there is a danger that data will be received before acquisition occurs, and therefore lost.
The problem can be minimized or corrected by increasing the size of the preamble, but that will degrade system throughput. Moreover, long preambles are also not possible in systems employing protocols such as Bluetooth which are based on standards requiring short preambles. Moreover, because of frequency hopping, in which different packets are transmitted at different frequencies, systems employing protocols such as Bluetooth cannot accommodate long preambles because of the adverse effect that would have on system throughput.
Conventional approaches also include open loop techniques such as that employed in PHS (Personal Handyphone System). An open loop system requires that the recipient be aware of an initial frequency estimation or initial timing estimation. These systems also require that the recipient be aware of the starting point of data transmission. Such a requirement is a significant limitation because it requires that all transmissions be synchronized, which, as a practical matter, may not possible in many systems today.
Some systems, such as TDMA systems, utilize burst detection techniques to detect the timing of an incoming transmission. However, many of these techniques are not reliable in the case of a short preamble as that employed in Bluetooth, which uses only 4 symbols in the preamble.